Multipath interference is generally caused when two or more signal rays of an original transmitted signal converge upon a receiving antenna of a receiver system at significantly different times. This misalignment or superposition of several delayed signals, which are generally replicas of the original signal, may cause distortion in audio recovered from the signals. Distortion caused by the multipath interference may be attributable to long delay (e.g., greater than five microseconds between signals) multipath interference or short delay (e.g., less than five microseconds between signals) multipath interference.
In various radio frequency (RF) receiver systems, antenna diversity has been implemented in conjunction with an FM receiver to reduce degraded reception performance caused by multipath interference. Antenna diversity has been accomplished through the use of two or more uncorrelated antennas. Conventional antenna diversity reception from mobile communication systems has been achieved by a number of different implementations. For example, antenna diversity has been accomplished with equal gain combiner (EGC) systems, maximal ratio combiner (MRC) systems, and antenna diversity systems, such as the adaptive reception system (ARS) disclosed in U.S. Pat. No. 5,517,686, the entire disclosure of which is hereby incorporated herein by reference.
EGC and MRC systems typically utilize signals from all antennas through a variety of combining techniques that attempt to optimize the certain characteristics of the received signals. In a switched antenna diversity system, only one antenna is generally utilized for reception at any instant in time and, thus, the non-selected antennas do not contribute to the demodulated signal. The EGC and MRC systems generally may provide superior performance, however, they also tend to be more expensive to implement and may require multiple receiver analog front ends.
One example of a conventional switched antenna diversity system 100 is illustrated in FIG. 1, which operates as a fast distortion detector. The system 100 generally includes a plurality of antennas 112A-112D coupled to a remote diversity module 114 which, in turn, is coupled to a single FM receiver 130 via two RF coaxial cables 128A and 128B. The FM receiver 130 typically includes front end circuitry 134, analog-to-digital conversion (ADC) circuitry 136, an FM demodulator 138, an audio processor 140 and a buffer 142.
The receiver 130 generates and outputs an analog intermediate frequency (IF) feedback signal at about 10.7 MHz and a DC (AM/FM mode) feedback signal to the diversity module 114 via RF coaxial cable 128B. The signals output from the receiver 130 are input to an analog FM demodulator and level detector 150 in the remote diversity module 114. The analog demodulator and level detector 150 outputs FM multiplex (MPX) and level detected (AM detected) signals to threshold comparators 152 and 154, the outputs of which are filtered by spike filter 156 and dip filter 158. Decision block 160 has intelligence that selects the next antenna via decision logic 120. Essentially, when a spike in the MPX signal and a negative dip in the level detected signal are detected, the next antenna is selected via decision logic 120 which causes activation of switches 116A-116D to select one of the antennas 112A-112D. The aforementioned conventional receiver system 100 generally employs costly intelligence in the remote diversity module 114 and requires two costly RF coaxial cables 128A and 128B.
Another conventional switched antenna diversity system 100′ is illustrated in FIG. 2 which employs a single RF coaxial cable 128, instead of the two coaxial cables, which reduces the cost of system 100′. Additionally, the FM receiver 130 employs a crossover network 170, and the remote diversity module 114 likewise employs a crossover network 172. The RF coaxial cable 128 transmits the RF signal, the IF signal and a DC control signal. The crossover networks 170 and 172 are added to both the receiver 130 and the remote diversity module 114 to separate the signals. In this system, the remote diversity module 114 likewise employs intelligence to determine the antenna selection. The conventional diversity modules generally require analog IF feedback signals to operate properly. In future systems, it is contemplated that low-IF digital FM receivers will be unable to adequately provide analog IF signals to the external diversity modules due to the complexity of the new receiver architecture.
It is therefore desirable to provide for a switched diversity module that interfaces properly with low-IF digital FM receivers. In particular, it is further desirable to provide for an RF receiver system and switched antenna diversity module that effectively controls selection of one of a plurality of antennas in a manner that reduces the multipath distortion in a mobile FM receiver.